1. Field of the Invention
The invention relates generally to electronic devices and processes for forming the same, and more specifically, to electronic devices including one or more guest materials within a layer and processes for forming the same.
2. Description of the Related Art
Electronic devices, including organic electronic devices, continue to be more extensively used in everyday life. Examples of organic electronic device include Organic Light-Emitting Diodes (“OLEDs”). Current research in the production of full color OLEDs is directed toward the development of cost effective, high throughput processes for producing color pixels. For the manufacture of monochromatic displays, spin-coating processes have been widely adopted. However, manufacture of full color displays usually requires certain modifications to procedures used in manufacture of monochromatic displays. For example, to make a display with full color images, each display pixel is divided into three subpixels, each emitting one of the three primary colors: red, green, and blue. This division of full-color pixels into three subpixels has resulted in a need to modify current processes for depositing different organic polymeric materials onto a single substrate during the manufacture of OLED displays.
One such process for depositing organic material layers on a substrate is ink-jet printing. Referring to FIG. 1, first electrodes 120 (e.g., anodes) are formed over a substrate 100. In addition, in order to form pixels and subpixels, a well structure 130 is formed on the substrate 100 to confine the ink drops to certain locations on the substrate 100. The well structure 130 typically is 1 to 5 microns thick and is made of an electrical insulator. A charge-transport layer 140 (e.g., a hole-transport layer) and organic active layer 150 may be formed by sequentially ink-jet printing each of the layers 140 and 150 over the first electrodes 120.
One or more guest materials may or may not be mixed with the organic active layer 150. For example, the organic active layer 150 within openings of the well structure 130 closest to the left-hand side of FIG. 1 may include a red guest material, and the organic active layer 150 within openings of the well structure 130 near the center of FIG. 1 may include a green guest material, and the organic active layer 150 within openings of the well structure 130 closest to the right-hand side of FIG. 1 may include a blue guest material. The well structure 130 tends to reduce the aperture ratio of a display, and therefore, higher current is needed to achieve sufficient emission intensity as seen by a user of the display.
In an alternative process, the charge-transport layer 140 and organic active layer 150 may be formed with or without a well structure. Inks with different guest materials may be placed on regions of the organic active layer 150. The inks may include a conjugated polymer. After the ink is placed on the organic active layer 150, a diffusion step is performed to drive guest material from the overlying polymer into the organic active layer 150. A second electrode (not illustrated) is formed over the organic active layer 150 and the ink.
Many problems occur when using this process for organic electronic devices formed by such processes. First, most of the guest material does not diffuse into the organic active layer 150. Typically, 25% or less of the guest material from the ink is diffused into the organic active layer 150. Therefore most of the guest material lies outside the organic active layer 150.
Second, the electronic components formed using this ink diffusion process have poor efficiency. As a basis for comparison, the same host material (as the organic active layer 150) and guest material may be mixed before the organic active layer is formed over the substrate. The combination of the host material and guest material may be spin coated and subsequently processed to form an electronic component. The spin-coated electronic component will be referred to as a corresponding conventional electronic component because the organic active layer has the same host material and guest material as the diffused component. Electronic components formed by the ink diffusion process have efficiencies that are lower than their corresponding conventional electronic components. Due to lower efficiency, the electronic components formed using the ink diffusion process have intensities too low to be used for commercially-sold displays.
Third, this ink diffusion process causes a very non-uniform distribution of guest material concentration, resulting in a high concentration gradient (change in concentration divided by distance) between electrodes with an electronic device. The guest material concentration within the organic active layer 150 near the second electrode is typically at least two and usually several orders of magnitude higher than the guest material concentration within the organic active layer 150 near the first electrodes 120. The high guest material concentration gradient makes the display nearly impossible to use, particularly over time. As the potential difference between the first and second electrodes are changed, the location for recombination of electrons and holes within the organic active layer 150 also changes, moving closer to or further from first electrodes 120 (depending on the relative change in potential difference). When the recombination is closer to the second electrode, more guest material is present at the recombination location. When the recombination is closer to the first electrode 120, less guest material is present at the recombination location.
This guest material concentration gradient in the organic active layer 150 causes a different spectrum to be emitted from the electronic component as the potential difference between the first and second electrodes changes. Note that higher intensity is typically achieved by increasing the current, which in turn typically occurs by increasing the potential difference between the first and second electrodes. Therefore, intensity control of a single color (i.e., “gray-scale”) is difficult because the emission spectrum shifts with a change in intensity, both of which are caused by a change in the potential difference between the first and second electrodes.
As a component ages, the amount of current needed for the same intensity typically increases. If the host material is capable of emitting blue light, as the intensity decays over time and current is increased (to try to keep intensity relatively constant over time), the emission of red and green doped pixels may become more blue with respect to their initial characteristic emission. As the component ages the operating voltage is increased to maintain constant luminance. If the guest material concentration profile in the host material is not sufficiently uniform, then as the operating voltage is increased to offset luminance decay, the emission spectrum of the red and green pixels may begin to include a blue component as a result of changes in the recombination zone position and width within the emissive layer.
Fourth, the ink diffusion process is nearly impossible to use in manufacturing because of the sensitivity to thickness of the organic active layer 150. Relatively small changes in thickness can have a large impact on the guest material concentration profile within the organic active layer 150. For displays, a user will observe variation from display to display, or even within the array of a single display, due to variation in the thickness of the organic active layer 150 during the fabrication process.
A different conventional process uses a vapor or solid phase diffusion process. Both processes suffer from similar problems previously described. If the diffusion is long enough to make the concentration of a guest material more uniform throughout a thickness of the layer (i.e., reduce the concentration gradient between the electrodes), lateral diffusion will be too large and can result in low resolution because the pixels will need to be large. Alternatively, if lateral diffusion can be kept at an acceptable level for high resolution, the guest material concentration gradient throughout the thickness of the organic layer may be unacceptably large. In some instances, both problems may occur (i.e., unacceptably large laterally diffusion while having too severe of a concentration gradient between the electrodes of the electronic device).